Scientific Focus The long-term goals of this project are to establish a molecular basis for the interdomain communication in glutamine amidotransferases that relates their dynamic properties to the control of their catalytic efficiencies. This investigation aims to delineate the major mechanistic and structural aspects of two enzymes, imidazole glycerol phosphate synthase (IGPS), and guanosine monophosphate synthetase (GMPS), which catalyze the transfer of ammonia from glutamine to a purine nucleotide acceptor substrate. The general hypothesis under consideration for the glutamine amidotransferase sub-family is that the functional link between the two active sites involves the transduction of binding and chemical energy from the acceptor active site to the glutaminase site. Furthermore, the glutamine substrate specificity and catalytic efficiency is controlled by dynamic protein-protein interactions that modulate a molecular tunnel for ammonia transfer. The proposed studies will focus upon structural and mechanistic information to define the domain interactions that control the timing of the nucleotide substrate activation and glutarnine hydrolysis steps. Significance of objectives Because of the unique chemical features present in the metabolic pathway under investigation, and the detailed comparative structural information that will emerge, this project has the potential to impact the discovery and development of new therapeutic agents. In addition, these enzymes represent important examples of convergent evolution that hold many mechanistic details of how proteins transmit specific chemical information over large distances. Overall, the combination of biochemical and biophysical approaches proposed will: a) contribute to a comprehensive knowledge regarding nitrogen metabolism and b) provide critical information for the design of new selective agents of potential pharmacological importance that target essential enzymes in ammonia metabolism.